1. Field of the Invention
Embodiments of the present invention relate generally to the optical detection of defects in disk storage media. In particular, embodiments of the present invention relate to a method and apparatus for optically detecting defects on the surface disk storage media using a differential amplitude modulation subsystem which produces an output signal with a very high signal-to-noise ratio to significantly enhance disk surface and surface event slope detection sensitivity.
2. Related Art
Disk drives typically employ one or more rotatable disks in combination with transducers supported for generally radial movement relative to the disks. Each transducer is maintained spaced apart from its associated disk, at a “flying height” governed by an air bearing caused by disk rotation. Present day transducer flying heights typically range from about 25 nm to about 50 nm, and experience velocities (relative to the disk, due to the disk rotation) in the range of 5-15 m/sec.
Effective recording and reading of data depend in part upon maintaining the desired transducer/disk spacing. Currently the amount of data that can be stored on the disk (i.e., the aerial density) is of great concern. As the aerial density increases and the flying height decreases, various surface defects in an otherwise planar disk surface of ever shrinking size become more and more significant. Thus, these defects or flaws can interfere with reading and recording, and present a risk of damage to the transducer, the disk recording surface, or both.
Therefore, the need arises for optically detecting, discriminating and measuring axial run-out and defects such as lumps, pits, scratches, micro-events, particles, etc. on the surface of disk storage media.